The point of doing an operating system UI in a game engine is so that you can use that operating system UI within games written in that engine!

There's several examples - Simulacra, Emily was Away, Hypnospace Outlaw - that use the interface of an (imaginary) computer as their primary framing device.

Or it could be useful in an immersive-sim type game where there happen to be computers you can interact with; you could immediately see the applications in some kind of hacking-themed game.